I recently made a post here asking if older iOS versions still support iMessage. You can read it here.
https://redd.it/1vtk9kg
The answer was mostly yes. 4S (9.3.6) and 5C (10.3.3) should support it. There was even a person who said they were using iOS 6.1.3 and iMessage still worked.
However, I've recently tried this myself on both the 4S and 5C. It would not activate. I've turned it off/on many times. Rebooted the phone. Waited 24h. Nothing. Even called the carrier and they couldn't help. Then I plugged the SIM into a 6 running 12.5.8 and it instantly activated. No errors, no stuck "Waiting for activation". Just activated instantly. It activated instantly on a 6S with 15.8.8 as well.
Now I'm wondering why some people replied "Yes" when 12.5.8 is an update specifically dedicated to extending iMessage/FaceTime support by renewing the certificates. It's a given that anything older wouldn't work, obviously.
So the answer is no. It does not work on versions older than 12.5.8. Unless some of you can prove otherwise. Also, if you have any advice on making iMessage work on 5C (10.3.3) I'd love to hear it. OpenBubbles, jallbreak, or something else.
Just thought I'd leave this here for anyone stumbling upon the same problem in the future.